In a UMTS (Universal Mobile Telecommunications System) network, attempts are made to optimize features of the system, which are based on W-CDMA (Wideband Code Division Multiple Access), by adopting HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access), for the purposes of improving spectral efficiency and improving the data rates. With this UMTS network, LTE (Long-Term Evolution) is under study for the purposes of further increasing high-speed data rates, providing low delay, and so on (non-patent literature 1).
In a third-generation system, it is possible to achieve a transmission rate of maximum approximately 2 Mbps on the downlink by using a fixed band of approximately 5 MHz. In a system of the LTE system, it is possible to achieve a transmission rate of about maximum 300 Mbps on the downlink and about 75 Mbps on the uplink by using a variable band which ranges from 1.4 MHz to 20 MHz. With the UMTS network, successor systems of LTE are also under study for the purposes of achieving further broadbandization and higher speed (for example, LTE-advanced (“LTE-A”)).
In the downlink of the LTE system (for example, Rel-8 LTE), CRSs (Cell-specific Reference Signals) that are associated with cell IDs are defined. The CRSs are used to demodulate user data, and, besides, used to measure downlink channel quality (CQI: Channel Quality Indicator) for scheduling and adaptive control, and so on. In the downlink of the LTE-A system (for example, Rel-10 LTE), which is a successor system of LTE, CSI-RSs (Channel State Information-Reference Signals) are under study, as reference signals for measuring channel state information (CSI). The CSI-RSs can support channel quality measurements of a plurality of cells, taking into account transmission and reception of signals between a plurality of cells. A user terminal feeds back channel state information to radio base stations, and the radio base stations control scheduling, adaptive radio link control, the number of layers to transmit and so on, based on the channel state information.
In the LTE-A system, carrier aggregation (CA) to achieve broadbandization by aggregating a plurality of fundamental frequency blocks (component carriers (“CCs”)) of different frequency bands is under study. With the LTE-A system, an agreement has been reached to make a single fundamental frequency block a frequency band that can be used in the LTE system (for example, 20 MHz), in order to realize broadbandization while maintaining backward compatibility with the LTE system. For example, when five fundamental frequency blocks (cells) are aggregated, the system band becomes 100 MHz.